In the router, while routing on the right mouse button you will find:
Tidy respects vias - turn this on. (this ensures that any route tidying will stop at the via so pad-route-via-copper will stop at pad-route-via when copper is removed.
Ring Removal - turn this off (this removes rings, if you have a GND connection that creates a ring this too can be removed).
Your powerplanes are actually set as electrical (they have to be otherwise you cannot pour/clear the template) and when they are cleared as they no longer have an automatic connection to a plane they are at risk of being seen as a ring connection, ring removal can remove this.
All the tools within CADSTAR have their place, situations when they need to be turned on and used, but also they have situations when they should be turned off - this is one of them for ring removal.